1. Field of the Invention
The invention relates to a fluid metering device that employs a sealing element in which a pressurized fluid, such as, a liquid or a gas, can be delivered or injected in a metered dose amount by the controlled and guided movement of a valve needle.
2. Description of the Prior Art
Various sealing elements or leadthrough elements are generally known in the art. However, in the case of the application of metering fuels that are under pressure of up to 300 bar, for example, and a work temperature range of -40.degree. C. up to +150.degree. C., special requirements have to be met by a product that is suitable for mass production. Generally, requirements have to be met concerning brittleness, wear and reliability.
The fatigue strength or wear over time of the known O-ring seals does not meet the above requirements. Instead of O-rings, membrane seals, such as, metal beads or other like membrane seals, can also be used. However, these have the disadvantage of having a highly pressure-loaded surface that effects the movement of the valve needle. Taking a 1 mm.sup.2 large pressure-loaded surface into consideration given a one-sided excess pressure of 300 bar, for example, a force of already 30 N results. Therefore, the utilization of membranes as a leadthrough or sealing element of the valve needle through a pressurized chamber cannot meet the requirements regarding a high axial elasticity or resilience and a sufficient resistance to pressure at the same time. Although a high resistance to pressure can be accounted for by a correspondingly dimensioned membrane, the increased membrane thickness results in a high axial rigidity. A pressure-independent function of the fluid metering device is not possible due to the large membrane surface and the extremely high pressure forces acting on the valve needle as a result thereof. However, a compensating member, such as, a mechanical spring, can be utilized in combination with the membrane seal to compensate or dampen the force that is directed to the valve needle via the membrane seal. Yet, the compensating member has limited use where it operates best in a single operating point.
It is also generally known that the valve needle leadthrough or sealing element can also be constructed of a clearance fit of the needle by way a cylindrical housing borehole. However, disadvantages result due to the unavoidable leakage along the needle leadthrough, so that a return line to the tank or to the low-pressure connection of the fuel feed pump is required. In addition, the overall efficiency of the motor is reduced as a result of the greater hydraulic losses.